At the nanoscale, alumina (Al2O3) shows promise in many applications, ranging from high value areas such as cancer therapy and transparent armor to more conventional areas such as polishing abrasives and cutting tools because of its unique mechanical, optical, and electronic properties. See H. Li et al., “Alpha-alumina nanoparticles induce efficient autophagy-dependent cross-presentation and potent antitumour response,” Nat Nano 6, 645-650 (2011); A. Krell et al., “Transparent Sintered Corundum with High Hardness and Strength,” Journal of the American Ceramic Society 86, 12-18 (2003); and H. Lei et al., “Preparation of alumina/silica core-shell abrasives and their CMP behavior,” Applied Surface Science 253, 8754-8761 (2007); see also U.S. Pat. No. 5,782,940 to P. S. Jayan et al., “Process for the preparation of alumina abrasives” (1998).
For example, alpha-alumina (α-Al2O3), more commonly known as corundum or sapphire, is one of the hardest known oxides behind only stishovite (a high pressure tetragonal SiO2 phase) and boron sub-oxide (B6O, an oxygen deficient metalloid). Dense nano-grained α-Al2O3 ceramics are theorized to have hardness values substantially higher than those for single crystals while maintaining high in-line visible light transmission. See J. A. Wollmershauser, et al., “An extended hardness limit in bulk nanoceramics,” Acta Materialia 69, 9-16 (2014); M. A. Meyers, “Mechanical properties of nanocrystalline materials,” Progress in Materials Science 51, 427-556 (2006); and R. Apetz, “Transparent Alumina: A Light-Scattering Model,” Journal of the American Ceramic Society 86, 480-486 (2003).
However, the crystal structure of alumina at the nanoscale depends on the crystallite size of the nanoparticle; consequently, the mechanical, optical, and electronic properties can vary dramatically and non-monotonically with particle size. See J. M. McHale et al., “Surface Energies and Thermodynamic Phase Stability in Nanocrystalline Aluminas,” Science 277, 788 (1997); and A. H. Tavakoli et al., “Amorphous Alumina Nanoparticles: Structure, Surface Energy, and Thermodynamic Phase Stability,” The Journal of Physical Chemistry C 117, 17123-17130 (2013).
The origin of the crystal structure versus crystallite size relationship is found to be contingent on the surface energy of the crystal structure. See McHale, supra. Therefore, the various synthesis routes which control the particle size result in the formed crystallites adopting one of the various crystal structures of alumina (including γ, δ, χ, η, θ, κ, and α), each with their own set of symmetry-dependent properties. See J. F. Nye, Physical Properties of Crystals: Their Representation by Tensors and Matrices, pp. xv-xvi (St Edmundsbury Press Ltd., 2004).
At the nanoscale, thermodynamics generally drives the crystal structure to alpha- (α-), gamma- (γ-), or amorphous alumina (a-Al2O3). Theoretical and experimental studies show that α-Al2O3 has a significantly higher surface energy (2.64 J/m2) than γ-Al2O3 (1.67 J/m2) or a-Al2O3 (0.97 J/m2). See A. Navrotsky, “Energetics of nanoparticle oxides: interplay between surface energy and polymorphism,” Geochem. Trans. 2003, 4(6), 34-37.
FIGS. 1A and 1B are plots illustrating the relationship of specific surface area (FIG. 1A) and particle size (FIG. 1B) to the thermodynamic stability of α-Al2O3, γ-Al2O3), and amorphous alumina (a-Al2O3).
Different surface energies are known to stabilize different polymorphs, see Navrotsky, supra, and, as particle size is reduced, the increase in specific surface area relative to volume changes the relative energy of finite sized crystal particles. As illustrated in FIG. 1A, the high surface energy of α-Al2O3 relative to γ-Al2O3 causes α-Al2O3 to become thermodynamically unstable, i.e., to have higher excess enthalpy with respect to γ-Al2O3 when the Al2O3 material has specific surface areas larger than ˜100-130 m2/g, see McHale, supra, and Tavakoli, supra, such that particles having such surface areas undergo a phase change from α-Al2O3 to γ-Al2O3, Similarly, the relatively high surface energy of γ-Al2O3 causes it to become unstable with respect to a-Al2O3 at specific surface areas larger than ˜370 m2/g (see FIG. 1A), producing a phase change from γ-Al2O3 to a-Al2O3 in particles of that size. Since the ratio of atomic surface to atomic volume increases as the particle size is reduced, these thermodynamic determinations point to size-dependent effects at the nanoscale. Assuming a spherical particle shape whose size equals the crystal size, the surface area of Al2O3 nanoparticles larger than ˜11-12 nm should cause such particles to have an alpha structure, while Al2O3 nanoparticles smaller than ˜3-5 nm will be amorphous (see FIG. 1B). Alumina nanoparticles from 5-11 nm adopt the gamma structure.
However, synthesis of nanocrystalline Al2O3 typically results in γ-Al2O3 when the crystallite size is ˜20-50 nm and α-Al2O3 when the crystal size is >50 nm.
Alpha-alumina particle sizes larger than those predicted by pure thermodynamic calculations are likely the result of the fast coarsening kinetics of α-Al2O3. See McHale, supra. Bottom-up synthesis techniques produce small a-Al2O3 or γ-Al2O3 particles which are very stable, even at high temperatures, due to the high energy barrier to α-Al2O3 nucleation. See McHale, supra, and Tavakoli, supra. Converting such particles to α-Al2O3 requires high temperatures, often in excess of ˜1000° C. (see G. P. Johnston et al., “Reactive Laser Ablation Synthesis of Nanosize Alumina Powder,” Journal of the American Ceramic Society 75, 3293-3298 (1992); and S. Pu et al., “Disperse fine equiaxed alpha alumina nanoparticles with narrow size distribution synthesized by selective corrosion and coagulation separation,” Scientific Reports 5, 11575 (2015)). However, α-Al2O3 is known to rapidly coarsen, i.e., develop larger particle sizes, at temperatures above 500° C., see McHale, supra, so that the temperatures needed for phase transformation will cause the small nanostructure to be lost. Thus, purely bottom-up synthesis techniques will inevitably eventually produce α-Al2O3 particles having crystallite sizes much larger than idealized in theoretical considerations.
Despite the difficulties in producing small nanocrystalline α-Al2O3, many works have claimed to find intricate and novel methods to synthesize bulk amounts of small α-Al2O3 nanoparticles. Karagedov et al. synthesized ˜25 nm α-Al2O3 nanoparticles using a ball mill with an unidentified “grinding catalyst.” See G. R. Karagedov et al., “Preparation and sintering of nanosized α-Al2O3 powder,” Nanostructured Materials 11, 559-572 (1999). Yoo et al. used AlCl3 with vapor phase hydrolysis to make an alumina precursors to be calcined into α-Al2O3 that had particle sizes of about 35 nm. See Y. S. Yoo et al., “Preparation of α-alumina nanoparticles via vapor-phase hydrolysis of AlCl3,” Materials Letters 63, 1844-1846 (2009). Borsella et al. used laser synthesis from gaseous precursors to synthesize 15-20 nm α-Al2O3. See Borsella et al., “Laser-driven synthesis of nanocrystalline alumina powders from gas-phase precursors,” Applied Physics Letters 63, 1345-1347 (1993). Zhang et al. calcined boehmite (γ-AlO(OH)) at 1000° C. to obtained rod like α-Al2O3 with sizes of 15 nm by 150 nm. See X. Zhang et al., “Nanocrystalline α-Al2O3 with novel morphology at 1000° C.,” Journal of Materials Chemistry 18, 2423-2425 (2008). Laine et al. utilized a liquid-feed flame spray pyrolysis to convert mixtures of intermediate alumina phases into α-Al2O3 with the smallest size being 30 nm. See R. M. Laine et al., “Nano α-Al2O3 by liquid-feed flame spray pyrolysis,” Nat Mater 5, 710-712 (2006). Das et al. used thermal decomposition of an aqueous solution of aluminum nitrate and sucrose which was subsequently calcined to get a 20 nm sized particles. See R. N. Das et al., “Nanocrystalline α-Al2O3 Using Sucrose,” Journal of the American Ceramic Society 84, 2421-2423 (2001).
However, these methods still produce α-Al2O3 larger than the thermodynamic limit of ˜12 nm, which reinforces the assumption that 12 nm is a lower limit to the size of an α-Al2O3 nanoparticle.
A very recent work has demonstrated the possibility that Al2O3 can be coerced into the alpha structure below 12 nm. Pu et al. used a low yield selective corrosion and refined fractionated coagulation separation technique to synthesize 10 nm α-Al2O3. See Pu, supra. The Pu technique used Fe to stabilize the surface energy of α-Al2O3 and precipitated α-Al2O3 within a solid Fe grain. Though high nanoparticle yield and ease of industrial scale-up are not practical for such an approach, it demonstrates the possibility of metastability of α-Al2O3 below the thermodynamic size limit.
However, all of these prior methods rely on expensive and unique machines that produce limited quantities of powder which hinders those techniques from leading the industrial production of nanocrystalline α-Al2O3 at or below the thermodynamics size limit.